A composite article may be formed of multiple layers of a reinforcing material impregnated with a thermoplastic or heat fusible resin. Each layer that makes up the composite is generally separately impregnated with resin prior to layup, forming a "prepreg". After forming the article from multiple layers of prepreg, the article is consolidated by subjecting it to heat and/or pressure which causes the resin to fully penetrate and join all layers of the prepreg together. If the resin employed is of the thermoplastic type, then the article is simply cooled to harden the resin to produce the fully consolidated article.
One group of thermoplastic resins of interest includes a poly(amide-imide) polymer. The amide component of the polymer contributes strength and flexibility to finished composites. The imide component lends thermal stability at relatively high temperatures, on the order of 500.degree. F. or more. The poly(amide-imide) resin is particularly useful for forming composites suitable for use in aircraft surfaces or in aerospace components that are subjected to high operating temperatures.
In forming prepregs suitable for layering to make consolidated articles, a poly(amide-imide) resin is first diluted in an organic solvent such as N-methyl-2pyrrolidone (NMP). The resin, which may include up to about 50% solvent, is then impregnated into a reinforcing material such as a woven fabric of carbon fibers, by dipping the material into a bath of the resin solution. After the reinforcing material leaves the resin dip tank, it is directed between two opposing rollers that squeeze out excess resin. Residual solvent is then removed by passing the soaked reinforcing material through a drying oven operated at a relatively low temperature. Temperature levels are typically held to low levels to avoid causing the resin to advance in molecular weight, known to adversely affect processibility in subsequent formation into composite articles. The low temperatures employed are generally only capable of removing solvent such as NMP to a residual content of about 4% by weight of the impregnated material.
Composite products of thermoplastic poly(amide-imide) resins are typically characterized by maximum operating temperatures that are generally substantially less than would be expected in light of the melting temperature of the pure resin. The presence of residual organic solvents, used in forming the prepreg has been determined to have a profound adverse effect upon the maximum use temperature of these poly(amide-imide) composites. For example, residual N-methyl-2-pyrrolidone solvent in a composite laminant will lower the glass transition temperature and consequently the maximum temperature at which the composite may be employed by as much as 100.degree. F.
Prior workers have found that removing solvent during composite article consolidation operations is extremely difficult. Processes that employ heating cycles that attempt to boil off the solvent at relatively low temperature under vacuum have not been successful because of the low diffusively of the solvent from the assembled composite article.
Where higher temperatures are employed, there is the danger of advancement of the molecular weight of the resin. The resin is extremely viscous and if the molecular weight of the resin is advanced significantly by exposure to high temperature, the polymer matrix simply will not flow to fully impregnate and join together the layered fibrous reinforcing materials.
The difficulty of solvent removal has long been of concern in an effort to produce void-free composites of resins that advance or cure at temperatures that limit solvent removal conditions. The difficulty is particularly evident in polyimide systems, for example, that may evolve significant water when the polymer is produced. Thus, Lubowitz et al. in U.S. 3,565,549 and U.S. 3,697,308 employ various heating sequences to evaporate solvents used in forming a polyimide prepreg, but still produces laminants that include on the order of 2% volatiles, resulting in a substantial reduction in strength of the finished composite. Pike in U.S. 4,601,945 forms composite articles from aqueous solutions of polyimide precursors. While achieving a 0.7% void volume content by employing staged heating with low pressure intervals to allow reaction water to escape, the process is time consuming.
There remains a need in the technical field for methods of producing prepregs that include a reduced volatile content such that final composites are substantially void-free and thus attain a maximum strength article having predictable and reliable strength characteristics.